1. Field of the Invention
The present invention relates to a manufacturing method of a solid electrolytic capacitor, and more particularly to a manufacturing method of a solid electrolytic capacitor using an electroconductive polymer as a solid electrolyte layer.
2. Description of the Related Art
In recent years, due to the miniaturization, high-speed operation, and digitalization of electronic devices, there has been a strong demand for capacitors which are small in size, have a high capacitance, and have a low impedance in high-frequency region in a field of a solid electrolyte capacitor.
Usually, a solid electrolytic capacitor has a solid electrolyte layer such as manganese dioxide or lead dioxide on a dielectric film which is formed by oxidizing the surface of a porous anode body. The porous anode body is made by sintering a molded body of valve metal powder, such as tantalum or aluminum. However, there has been a problem in using this solid electrolyte layer for capacitors in high frequency region because it has a high resistance itself and also has a high equivalent series resistance and high impedance in high-frequency region.
To improve the characteristics for the conventional solid electrolytic capacitor in high frequency region, is known a capacitor with a solid electrolyte layer of an electroconductive polymer such as a five-membered heterocyclic compound (e.g., polypyrrole and polythiophene) and polyaniline, which has a higher conductivity compared with that for the conventional solid electrolyte layer. This electroconductive polymer as a solid electrolyte layer is formed by chemical or electrochemical oxidative polymerization on the dielectric film such as Ta2O5 made by anodizing electrochemically on the surface of the porous anode body.
However, it is difficult to form an electroconductive polymer layer on the entire surface of the pores of a porous body element covered with a dielectric film. Because of this, the capacitance appearance factor decreases and the equivalent series resistance increases in a high-frequency region for the solid electrolytic capacitor with an electroconductive polymer electrolyte.
FIGS. 1A and 1B are sectional views of a small pore of the porous body element of a solid electrolytic capacitor to explain the formation of the electroconductive polymer layer in its pore by chemical oxidative polymerization after the formation of the dielectric in a conventional manufacturing method of a solid electrolytic capacitor with the electroconductive polymer electrolyte.
In the conventional method, as shown in FIG. 1A, a porous body element 6 with a dielectric film 3 formed on the surface is soaked in an oxidant solution (not shown in FIG. 1A) to make the inside and the outside of a pore 10 of the porous body element 6 adsorb the oxidant. Then, the porous body element is soaked in a monomer solution. When the porous body element is soaked in the monomer solution, chemical oxidative polymerization starts and an electroconductive polymer layer 11, an electrolyte material, is formed before the monomer permeates enough into the pore 10 of the porous body element 6. As a result, as shown in FIG. 1B, the entrance of the pore 10 of the porous body element 6 is clogged with the electroconductive polymer layer 11, preventing the oxidant and the monomer from permeating into the pore 10 of the porous body element 6 in the following chemical oxidative polymerization process.
A technique, which promotes a chemical oxidative polymerization reaction of the monomer for forming the electroconductive polymer layer at the pore and improves the capacitance appearance factor of the capacitor, is disclosed in Japanese Patent Applications Laid-Open No. 11-74157, No. 11-219862, and No. 2000-21686.
In the technique disclosed in Japanese Patent Application Laid-Open No. 11-74157, an electroconductive polymer layer is formed by a chemical oxidative polymerization as follows. A porous body of an electrochemical valve metal with a dielectric film formed on the surface is soaked in a solution (hereinafter referred to as a xe2x80x9creaction solutionxe2x80x9d) which contains a monomer, an oxidizing agent, and a dopant. After the porous body is soaked for a predetermined time, an electroconductive polymer layer is formed on the surface of the dielectric film in the reaction solution. Or, a porous body of an electrochemical valve metal with a dielectric film formed on the surface is soaked in the reaction solution and pulled out of the reaction solution after some of the reaction solution is adsorbed on the surface of the dielectric film. Then, chemical oxidative polymerization takes place in the presence of air and an electroconductive polymer layer is formed on the surface of the dielectric film. While the electroconductive polymer layer is formed on the surface of the dielectric film, the temperature of the porous body itself or of the spaces inside the pores is kept higher than that of the solution. In this technique, by keeping the porous body or inside temperature higher than the solution temperature, the chemical oxidative polymerization reaction rate inside the porous body becomes faster than outside and the production of the electroconductive polymer layer inside a small pore of the porous body is enhanced.
However, in this technique, there is a problem that the production of the electroconductive polymer layer is not uniform inside the small pores because it is difficult to control the temperature inside the pores of the porous body stabilizing.
In the technique disclosed in Japanese Patent Application Laid-Open No. 11-219862, by oxidizing a sintered electrochemical valve metal, a porous body with an oxide film (a dielectric film) on the surface is formed. The porous body is soaked in a monomer solution and then in an oxidant solution to form an electroconductive polymer layer on the oxide film. To improve the yield of the electroconductive polymer layer inside the pores of the porous body, the soaking time of the porous body in the oxidant solution after soaking in the monomer solution is kept less than the time for 30% of the monomer in the porous body to flow out by diffusion. And also the reaction temperature is decreased. As a result, the monomer outflow from the pores is decreased and the yield of the electroconductive polymer layer inside the pores of the porous body is increased.
In this technique, there is a problem that the yield of the electroconductive polymer layer is not uniform inside the pores of the porous body and the capacitance of the capacitor becomes unstable because it is difficult to control the flow of the monomer out of the porous body to a certain constant amount.
In the technique disclosed in Japanese Patent Application Laid-Open No. 2000-21686, the outside of the porous body (dielectric film) is covered with the first electroconductive polymer layer and then the second electroconductive polymer layer is formed inside the dielectric film. The first electroconductive polymer layer is formed in a solution with a higher concentration of the oxidizing agent than the second electroconductive polymer layer so that a high polymerization rate is achieved and the electroconductive polymer layer can be formed preferentially on the surface except small pores, the outer surface of the dielectric film.
In this technique, there is a problem that the first electroconductive polymer layer prevents the solution for forming the second electroconductive polymer layer inside small pores from permeating, and as a result, it restricts the formation of the electroconductive polymer layer inside small pores because the outer surface of the porous body is already covered with the first layer.
Accordingly, an object of the present invention is to provide a manufacturing method of a solid electrolytic capacitor using an electroconductive polymer as an electrolyte material which improves the capacitance appearance factor and equivalent series resistance in high frequency region.
The manufacturing method of a solid electrolytic capacitor is characterized by comprising the steps of: contacting a dielectric film of a porous body made of an oxide film of a electrochemical valve metal with an oxidant solution; and contacting the dielectric film with an electroconductive polymer forming monomer solution to form the electrolyte layer made of the electroconductive polymer on the surface of the dielectric film by an oxidative polymerization reaction, wherein an oxidative polymerization retarding agent which delays the oxidative polymerization reaction, is added to at least one of the solutions, an oxidant solution and a monomer solution.
The oxidative polymerization retarding agent is an additive to prevent the chemically-oxidative polymerization from occurring immediately when the oxidant and the monomer contact each other and to delay starting of the chemical oxidative polymerization reaction.
Pyrrole, pyrrole derivatives, thiophene, tiophene derivatives, or aniline is used as a monomer. Oxime compounds nitro compounds, nitroso compounds, nitroxide compounds, chinone compounds, or phenol compounds is used as an oxidative polymerization retarding agent.
Iron(II) sulfonate, sulfuric acid, or hydrogen peroxide is used as an oxidizing agent.
In the present invention, when an electrolyte layer made of an electroconductive polymer is formed by a chemical oxidative polymerization reaction, the starting time of the polymerization to form an electroconductive polymer layer is delayed by using an oxidative polymerization retarding agent. Because the polymerization started after the monomer and the oxidizing agent solution have permeated well into small pores, the electroconductive polymer layer is formed inside small pores and the capacitance appearance factor and the equivalent series resistance of a solid electrolytic capacitor with an electroconductive polymer as an electrolyte material are improved.